The invention relates to a blocking converter particularly suited for secondary-cell or battery driven photographic flash equipment having a switching transistor and a transformer whose primary winding is connected in the collector circuit of the switching transistor, and whose secondary winding charges a charging capacitor via a rectifier diode. A rectifier diode is connected to the transformer secondary in such a manner that it only conveys current when the switching transistor is turned off. A trigger circuit periodically turns the switching transistor on and off.
Self-oscillating flow converters are at present mostly used in electronic flash equipment for the charging of the flash capacitors. Germanium transistors are employed as switching transistors due to their advantageous properties. Blocking converters have more favorable properties than flow converters, particularly a considerably higher efficiency. SInce, however, in electronic flash equipment they require a greater circuit expense than flow converters, they have not been commercially successful up to now.
Blocking converters operate in accordance with the following principle. A linearly rising current flows in the primary winding of the transformer during the on-time of the switching transistor. The energy stored in the transformer is conveyed to the charging (flash) capacitor during the subsequent blocking time of the transistor. The transistor operates continuously with the same comparatively low switching power so that an operation even with low power devices is possible. Even if short circuits occur at the output, the transistor cannot be overloaded. Electrolytic capacitors stored for a long time can therefore be formed in continuous operation without damage to the apparatus. Since the fly-back voltage is used for charging, the transmission ratio of the primary winding to the secondary winding can be selected smaller than the ratio of battery voltage to charging voltage.
If an automatic cutoff is additionally provided, then one always obtains the same charging voltage, even with a varying battery supply voltage, and thus the same energy is stored in the capacitor.
The blocking converters commonly used up to now are of the self-oscillating type, a control energy being produced via an additional feedback winding on the transformer. The switching transistor is biased in such a manner that it is conducting when the battery voltage is turned on. As soon as the transformer is saturated or the transistor saturation voltage rises, the control voltage induced in the feedback winding becomes zero. The transistor begins to block. The collapsing magnetic field in the transformer, produces a high voltage, the so-called fly-back voltage. As soon as the energy stored in the transformer is reversed into the charging capacitor, the blocking voltage induced in the feedback winding again becomes zero, whereby the switching transistor again becomes conducting, and again electric energy is stored in the transformer. The disadvantage in self-oscillating blocking converters is that without an individual adjustment, an optimal operating condition is not possible in each case.